In commonly used electrostatic printers, also known as direct electrostatic printers, the image is printed directly onto a special final medium which comprises a conductive substrate on which a dielectric layer a few micrometers thick is deposited. The image printing process is performed in several successive operations taking place in distinct stations for each operation. Essentially the process consists in three successive stages: a printing stage which takes place in a printing station and comprises creating an electrostatic latent image on the special medium by discharging ions thereon from a print head having multiple miniature electrodes arranged in some suitable pattern such as one or more rows or a matrix, said discharge passing through the air in a gap between the electrodes and the dielectric layer; then a developer stage which takes place in an image developer station and comprises inking the latent image with a deposit of fine particles of magnetic powder or ink using a magnetic brush or other inking means; and finally a fixing stage which takes place in an image fixing station and comprises fixing the image by application of heat, eg. by passing the medium through an oven.
Electrostatic printers of the type with which the present invention is concerned and known as indirect electrostatic printers also exist. In these printers, the electrostatic latent image is generated on an intermediate medium, is developed thereon, and is then transferred to a final medium, usually paper, on which it is fixed. The various printing stages or operations in an indirect electrostatic printer are performed by means analogous to those used in the successive stations of a direct electrostatic printer, with a transfer stage in which ink is transferred from the intermediate medium to the final medium being inserted in between the developer stage and the fixing stage. The transfer advantageously takes place in an image transfer station, by pressing the final medium of plain paper closely against the inked latent image on the intermediate medium, and is generally accompanied by the application of a suitable voltage to a transfer electrode disposed behind the final medium for enhancing said transfer.
In an indirect electrostatic printer, the intermediate medium may be constituted by a conductive substrate on which a thin dielectric layer is deposited, or alternatively, it may be constituted by a simple film of dielectric material.
Thus. U.S. Pat. No. 3,198,648 describes an electrostatic printing process in which for reasons of cost, a dielectric band is used as the intermediate medium and plain paper is used as the final medium. In the printer, the dielectric band is mounted in an endless loop passing over two rollers. One of the rollers is electrically conductive and constitutes a counter-electrode which is associated with a multiple electrode print head for printing the latent image on the outer surface of the dielectric band as it passes over the counter-electrode roller. The image is then inked by an image developer system located close to said counter-electrode roller.
At the other roller over which the dielectric band passes, there is a pressure roller which presses the final plain paper closely to the previously inked image to transfer the particles of ink onto the paper where they are subsequently fixed by applying greater pressure than is used to transfer the image. Finally the dielectric band is cleaned and electrostatically discharged after the image has been transferred therefrom in readiness to receive a further image as it passes over the counter-electrode roller.
Preferred embodiment of the present invention provide an indirect electrostatic printer using a flexible intermediate medium, such as a tape of dielectric material, which is of simple and compact structure, and which delivers printed images on sheets of a final medium such as paper.